Ablation is the removal of material from the surface of an object by vaporization, chipping or other erosive processes. The term “ablation” is often used in the context of laser ablation, a process by which laser beam energy is applied to the surface of a solid or liquid material. At least some of this energy is absorbed by the material, which can lead to several mechanisms of material removal, including: melting with further expulsion due to shock or to high-pressure assist gas, sublimation of the material directly into gas phase due to very fast heating, photo-chemical bond breaking resulting in the release of solid, liquid or gaseous fragments, or a combination of any of the foregoing. The type of ablation of a given material depends on the material's properties (e.g., density, absorption spectrum, thermal conductivity and diffusivity and/or specific heat) and the laser's characteristics (e.g., wavelength, pulse energy, pulse rate, pulse duration and/or beam fluence on target).
For a given ablation application, an important characteristic is the beam fluence on target (i.e., the energy density per unit area, usually measured in J/cm2). As the beam penetrates the material, its fluence is attenuated generally exponentially at the rate determined by the material's absorption coefficient at the laser's wavelength. For a given material, the ablation rate is defined as a thickness of a layer of a material which is removed by one pulse. In general, ablation occurs only when the beam fluence exceeds an ablation threshold (Fth). If the beam fluence is below the threshold, only heating of the material occurs without ablation. If the beam fluence is above the threshold, in general, the thickness of the material ablated, and thus the ablation rate, is determined by how far into the material the beam penetrates while still having beam fluence above Fth. Thus, ablation rate increases with fluence, and ranges typically, from ˜10 nm to ˜10 μm per pulse. Generally, pulses contribute cumulatively to ablation, i.e., the amount of ablated material (e.g., the depth in cases of drilling) is proportional to the number of pulses. In some situations, there are variations which may depend upon, for example, pulse rate or the shape and size of the channel being ablated. However, typically the more pulses applied, the more material that will be ablated.
The physics described above and the development of laser technology has resulted in wide use of laser ablation for advanced fabrication methods. Lasers are used for controlled removal of materials on different size scales, especially for micromachining down to sub-microns. Both direct-write and mask-projection techniques may be used, and laser beam wavelengths may be selected based on the materials being processed.
Laser ablation may be used to remove one or more layers of material from work pieces of various shapes and sizes. For example, laser ablation may be used to strip a layer of material of uniform thickness from the circumference of a substantially cylindrical work piece, such as a wire, catheter, tube or needle, by either rotating the work piece while exposing it to a stationary laser beam, keeping the work piece stationary and exposing it to multiple laser beams or some combination of both. Laser ablation may also be used to remove a uniform outer layer from a non-cylindrical work piece by either moving the work piece relative to a stationary laser beam, moving a laser beam relative to the work piece or a combination of both (e.g., the work piece may be scanned in one direction by moving the work piece and scanned in another direction by moving a laser beam).
When a layer of material has a substantially uniform and known thickness, and an ablation rate is known or can be determined, a laser ablation process may be pre-configured to remove only that layer of material. The ablation depth of the laser beam may be pre-configured by selecting appropriate parameters, such as wavelength, pulse number and fluence. However, when the thickness of the material is non-uniform and/or unknown, a pre-configured laser beam would ablate the same thickness of material along the entire work piece, leaving either uneven amounts of material on the work piece and/or cutting into one or more underlying layers of material on the work piece.
When the thickness of a layer of material is non-uniform and/or unknown, but it is known that the ablation threshold of a first layer, Fthfirst, is sufficiently lower than that of an adjacent underlying second layer, Fthsecond, a substantially complete removal of the first layer can be achieved by employing a beam having a fluence value, Fbeam, that is in between the ablation threshold values of the first and second layers, i.e., high enough to ablate the first layer and but too low to affect the second layer, or Fthfirst<Fbeam<Fthsecond. However, this favorable condition is not always available.
One of the challenges of laser ablation is therefore precisely removing one or more selected layers of material from a multi-layer work piece without ablating, damaging or otherwise altering other layers in the work piece. This challenge becomes particularly acute when attempting to remove a first layer of material from a second layer of material when the thickness of the first layer is non-uniform/asymmetrical and/or unknown and the ablation threshold values are not sufficiently different.